1. Field of the Invention
The present invention relates to a positive electrode active material for a lithium battery, a method for producing the same, and a battery containing the same in the positive electrode thereof.
2. Description of the Related Art
With the recent development of portable electronic equipment, batteries of higher performance have been demanded. Lithium ion batteries using a carbon material in the negative electrode and lithium cobaltate (LiCoO.sub.2), which is a composite oxide having a layer structure, in the positive electrode have been put to practical in a nonaqueous battery having a high working voltage and a high energy density. Lithium nickelate (Li-containing nickel oxide; LiNiO.sub.2) is also a compound having the same layered crystal structure as the lithium cobaltate in which lithium ions are intercalated between layers of NiO.sub.6 octahedral sharing edges.
Lithium nickelate is generally prepared by mixing a nickel source selected from Ni(NO.sub.3).sub.2, Ni(OH).sub.2, NiCO.sub.3, NiO, NiOOH, etc. and a lithium source selected from LiOH, LiNO.sub.3, Li.sub.2 CO.sub.3, Li.sub.2 O.sub.2, etc., and subjecting the mixture to a heat treatment at about 600 to 900.degree. C. in an oxygen stream.
Since cobalt or nickel used in these active materials is expensive for scarcity, less expensive active materials for a positive electrode has been sought. For example, Li-containing manganese composite oxide (LiMn.sub.2 O.sub.4) having a spinel structure has been proposed, but its theoretical capacity of 148 mAh/g is low, and the reduction in capacity increases with charge and discharge cycles.
LiMnO.sub.2 has been proposed as a promising active material for batteries with higher performance. Among various phases exhibited by LiMnO.sub.2, two phases, whose crystal structure have been well characterized, are a high temperature orthorhombic phase (Pmnm) and a low temperature tetragonal phase (I4.sub.1 /amd). Both structures involve cubic close packing but they differ in the arrangement of the ordering of the lithium and manganese cations. The tetragonal form Li.sub.2 Mn.sub.2 O.sub.4 is prepared by electrochemically or chemically intercalating lithium into the spinel LiMn.sub.2 O.sub.4 [Mat. Res. Bull, 18(1983)461 & 18(1983)1375; J. Electrochem. Soc. 138(1991)2864 & 139(1992)937]. The orthorhombic phase has been prepared mainly by the solid state reaction at high temperature using different precursors [J. Phys. Chem. Solid. 3(1957)20 & 318; J. Phys. Radium 20(1959)155; J. Anorg. Allg. Chem. 417(1975)1; Mater. Res. Bull. 28(1993)1249]. However, orthorhombic LiMnO.sub.2 was reported to be prepared at low temperature using the solid state reaction by heating a mixture of .gamma.-MnOOH and LiOH at 300 to 450.degree. C. [Chem. Express, 7(1992)193]. An other process for preparing the orthorhombic LiMnO.sub.2 at a temperature less than 100.degree. C. by ion exchange was reported. [J. Electrochem. Soc. 140(1993)3396; Unexamined Japanese Patent Publication (kokai) No. 6-349494] In this case, the exchange was carried out by refluxing .gamma.-MnOOH under boiling condition in LiOH solution. So far, LiMnO.sub.2 isostractural with layered LiNiO.sub.2 or LiCoO.sub.2 has not yet been synthesized.
Referring to LiMnO.sub.2 having a layer structure, J. Solid State Chem., 104(1993)464 and U.S. Pat. No. 5,153,081 report that LiMnO.sub.2 having a monoclinic layer structure can be obtained by acid leading of Li.sub.2 O out of Li.sub.2 MnO.sub.3. In the first step, Li.sub.2 MnO.sub.3 was prepared by reacting electrolytic manganese dioxide (EMD) with a stoichiometric quantity of Li.sub.2 O.sub.3. The obtained material was then delithiated using H.sub.2 SO.sub.4 at room temperature for 64 hours.
The thus obtained substance exhibits a discharge voltage of 3 V vs. Li/Li.sup.+. Although the reaction product exhibits a new X-ray diffraction peak at 2.theta.=19.5.degree., most of the other peaks correspond to the starting material which is Li.sub.2 MnO.sub.3 (U.S. Pat. No. 5,153,081). Ignoring the fact that most of the peaks are assigned to the starting Li.sub.2 MnO.sub.3, the inventors of U.S. Pat. No. 5,153,081 identify the product to be a substance having a layer structure based on the peak at 2.theta.=19.5.degree., but the identification seems to be decisively unreasonable. In this case, the product should rather be regarded as a lithium manganese oxide having a spinel structure as a basic skeleton, such as Li.sub.2 Mn.sub.4 O.sub.9 or Li.sub.4 Mn.sub.5 O.sub.12.
Further, J. Solid State Chem., 104(1993)464 reports a substance having a layer structure whose X-ray diffraction pattern is different from those of LiNiO.sub.2 or LiCoO.sub.2. While not entering into details about structural refinement of the substance, the report based their layered structure on the assumption that removal of Li.sub.2 O from Li.sub.2 MnO.sub.3 causes a shearing of the closed-packed oxygen planes to yield an oxygen array in the obtained material comprised of alternate layers of trigonal prisms where lithium is located and sheets of edge-shared octahedra where manganese is located. In this case, the manganese ions remain in alternate layers and do not migrate to the lithium layers during the leaching process, and the lithium layer is arranged in a zig-zag fashion with lithium ions in a trigonal prismatic coordination.
Unexamined Japanese Patent Publication (kokai) No. 7-223819 reports that LiMnO.sub.2 having a layer structure with a lattice constant of a=3.321 .ANG. and c=4.730 .ANG. is obtained by electrolysis method. This material is not isostructural with LiNiO.sub.2. Thus, no LiMnO.sub.2 having a layer structure similar to that of LiNiO.sub.2 or LiCoO.sub.2, has been synthesized yet.
As stated above, although 4.0 V type LiMn.sub.2 O.sub.4 having a spinel structure has been proposed as an inexpensive Li-containing manganese composite oxide, the theoretical capacity is inferior to oxide compounds having a hexagonal layer structure, such as LiNiO.sub.2 (theoretical capacity: 275 mAh/g) and LiCoO.sub.2 (theoretical capacity: 274 mAh/g). In addition, the charge and discharge cycle characteristics are better in layered oxide materials. Therefore, development of an inexpensive active material having a layer structure similar to that of LiNiO.sub.2 or LiCoO.sub.2 and establishment of synthesis therefore have been keenly demanded, but a useful method of synthesis has not yet been established.